insertHeader hdr val x inserts a header with the given header name and value. Does not check for existing headers with same name, allowing duplicates to be introduce (use replaceHeader if you want to avoid this.)

Deprecated. As of version 0.5, replaced by insertWith.
O(log n). Same as insertWith, but the value being inserted to the map is evaluated to WHNF beforehand.
For example, to update a counter:
> insertWith' (+) k 1 m

Construct a Builder that always inserts the strict ByteString directly as a chunk.
This implies flushing the output buffer, even if it contains just a single byte. You should therefore use byteStringInsert only for large (> 8kb) ByteStrings. Otherwise, the generated chunks are too fragmented to be processed efficiently afterwards.